A development trend of wireless communications towards broader bands requires that a duplexer at a radio frequency front end of a base station have a smaller volume, a larger power capacity, and lower costs, and be also capable of maintaining performance such as a loss. A cavity (that is, a coaxial resonant cavity filled with air) filter is a traditional technology of a base station duplexer, and the technology is mature and cost effective. The cavity filter generally includes a cover and multiple cavities, and multiple resonant tubes are disposed in each cavity. Each cavity functions as an electronic oscillator circuit, and when the filter is tuned to a proper wavelength of a received signal, the oscillation circuit may be represented as a parallel oscillation circuit that includes an inductance part and a capacitance part, and a resonant frequency of the filter may be adjusted by adjusting the inductance part or the capacitance part.
A method for adjusting a capacitance is to adjust a distance between a resonant tube and a cover, where the distance is generally adjusted by screwing a tuning screw in or screwing a tuning screw out of a screw hole on the cover. As a volume of a single cavity continuously decreases, a current density on a surface of the single cavity increases and a loss continuously increases. The decrease in the volume also reduces a distance between surfaces of conductors in the single cavity. As a result, electric field strength increases and finally exceeds an air breakdown threshold, thereby reducing a power capacity. Therefore, a smaller volume of the cavity filter leads to a larger loss and a smaller power capacity, which cannot meet a requirement of maintaining unchanged performance with a smaller volume.
The cavity filter generally uses a metal resonator, that is, the cavity, the resonant tube, and the like are all made up of a metal material or a material that is metallic at least on an inner surface. In a case in which a volume of a single cavity of a transverse magnetic (TM) mode dielectric filter is the same as a volume of a single cavity of the cavity filter, the TM mode dielectric filter uses high-performance ceramic resonator instead of a metal resonator. When a reduced conductor loss of the TM mode dielectric filter is greater than a dielectric loss brought by the TM mode dielectric filter, a smaller insertion loss can be achieved. In addition, because parts that have the highest electric field strength in the TM mode dielectric filter are centralized inside the medium, and breakdown field strength of a medium material is far higher than that of air, the power capacity can also be greatly improved. However, a high-performance ceramic material generally includes rare earth, and due to global scarcity of rare earth resources, a price of the high-performance ceramic material is high.